In recent years, flat panel displays such as liquid crystal displays (LCDs) have been widespread. In a display device such as a flat panel display, pixels are arranged in the row direction and the column direction, and each pixel includes a transistor serving as a switching element, a liquid crystal element electrically connected to the transistor, and a capacitor connected to the liquid crystal element in parallel.
A semiconductor film of the transistor is generally formed using a silicon semiconductor such as amorphous silicon or poly-silicon (polycrystalline silicon).
Metal oxides having semiconductor characteristics (hereinafter referred to as oxide semiconductors) can also be used for semiconductor films of transistors. For example, techniques for forming transistors including zinc oxide or an In—Ga—Zn-based oxide semiconductor are disclosed (see Patent Documents 1 and 2).